Optimum efficiency for these processes may be attained by designing a conven- 

 tional water treatment plant. However, it may be possible to achieve adequate 

 effluent treatment or pretreatment with a simpler system that has been demon- 

 strated on other dredging projects. Components needed for a simple chemical 

 clarification system for treating CDF effluent are weirs, corrugated metal 

 pipe or culvert, polymer pumps, and equipment to inject the polymer into the 

 effluent from the primary containment area. A secondary containment area pro- 

 vides the capacity for gravity settling of the coagulated suspended solids. 

 This section provides an example of the design approach for these auxiliary 

 components of a CDF. 

 Weir 



45. The purpose of the weir is to regulate the release of ponded water 

 from the containment area. To design a weir to regulate the ponded water, 

 several assumptions must be made. The dredged material slurry flow rate is 

 assumed to be 400 cu yd per hour or 3,200 cu yd per day for an effective pro- 

 duction time of 8 hr per day. The dredged material exhibits zone settling for 

 the untreated material entering the primary containment area and flocculent 

 settling for the treated material entering the secondary containment area. A 

 rectangular weir will be designed to maintain 2 ft of ponding depth in each 

 containment area. 



46. Figure 15 illustrates the relationship between effective weir 

 length and ponding depth necessary to discharge a given flow without entrain- 

 ing settled material (EM 1110-2-5027, USACE 1987). Using the assumptions men- 

 tioned earlier and Figure 15, construct a horizontal line at the design flow 

 of 3 cfs (400 cu yd/hr) and the 2-ft ponding depth. A vertical line drawn at 

 the point of intersection of the horizontal ponding depth line and the zone 

 settling operation line yields a weir loading rate of 0.85 cfs/ft (see Fig- 

 ure 16). For 3 cfs, a weir length of 3.5 ft is required. Because of current 

 uncertainties in the actual flow rates of the dredges to be selected and 

 because in the final stages a ponding depth of less than 2 ft may occur, a 

 safety factor of 2 will be applied to the design, yielding a weir length for 

 the primary area of 7 ft. 



47. A weir length of 8 ft is required for the secondary containment 

 area. The procedure is the same as above except flocculent settling will 

 occur instead of zone settling, and a deeper ponding depth (3 ft) could be 

 made available in the secondary area (Figure 16) . 



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